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Voltage gain in decibels is defined as:
G_dB=20*〖log〗_10 (Vout/Vin).
Positive gain means amplification, and negative gain indicates attenuation. So if a circuit had a Vout of 1 volts and a Vin of √2 volts (a voltage drop), its gain would be:
G_dB=20〖*log〗_10 (1/√2)=-3 dB.
This -3 dB mark is important, as it indicates where the circuit’s output power (not
voltage!) is exactly half of its input power.
The phase plot describes how different frequencies take relatively shorter or longer
time to travel through the circuit. Any frequency with a phase reading of -180? or –π
radians will be unstable at that frequency.
To create a Bode plot from an existing circuit, test the circuit with a range of frequencies.
This range depends on the application at hand, such as audio or data transmission.
Stimulate the circuit’s input with a simple sine wave at the frequencies of interest.
Measure the input and the output with an oscilloscope, and compare the difference
between the two. Record these differences in a spreadsheet, then graph them to see
the final Bode plot. The data can be tabulated and plotted by hand instead, if desired.
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